Dinitrogen tetraoxide-difluoroaminosubstituted fuel compositions and method of usingsame for rocket propulsion



United States Patent 3 383 859 -DINITROGEN TETRAOXiDE-DIFLUOROAMINO- SUBSTITUTED FUEL COMPOSITIONS AND METHOD OF USING SAME FOR ROCKET PROPULSION Edgar F. Croomes, Huntsville, Ala., assignor to the United States of America as represented by the Secretary or the Army No Drawing. Filed Sept. 10, 1962, Ser. No. 222,737

" 8 Claims. (Cl. 60-214) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to liquid propellant compositions useful for fueling rocket motors. Specifically, this invention concerns liquid composition based on difluoroaminosubstituted derivatives of organic compounds as fuels and dinitrogen tetraoxide as the oxidizer.

Recent efforts in the development of rocket propellant systems have been directed to the achievement of higher specific impulse values than those obtainable with fuels in current use. Theoretical calculations have shown that systems based on carbon-hydrogen-oxygen-nitrogen-fluorine propellants can produce an impulse of 280 lb.-sec./lb. or greater while those systems based on carbon-hydrogenoxygen-nitrogen-propellants are limited to 250 lb.-sec./lb.

Compounds which appear to be particularly useful in propellant compositions due to their ease of synthesis and superior performance are those containing the difluoroamino group, --NF One disadvantage encountered in the utilization of these fuels has been their sensitivity to shock resulting in the detonation of the compounds and propellant compositions containing them. This sensitivity to shock refers to the sensitivity the compounds exhibit towards sudden impact such as experienced when the compounds or their containers are dropped, struck or jarred.

' Due to the sensitivity of the compounds, special handling techniques are required. This is time consuming and expensive. Moreover, for military purposes, propellant shock sensitivity and the problems associated therewith may eliminate the propellant from consideration for obvious reasons.

It has now been determined that difluoroamino-substituted liquid propellants employed in conjunction with dinitrogen tetraoxide as the oxidizer exhibit a markedly decreased sensitivity to shock while affording liquid propellant compositions of high specific impulse values. This is a particularly unexpected result in view of the fact that the difluoroamino-substituted compounds thus far tested do not perform acceptably as propellant fuels when conventional liquid oxidizers such as liquid oxygen and hydrogen peroxide are employed. Moreover, the difluoroamino derivatives maintain their sensitivity in the presence of these other conventional oxidizers.

It is, therefore, an object of this invention to provide improved liquid propellant compositions based on difiuoroamino-substituted compounds as fuels and dinitrogen tetraoxide as the oxidizer, said compositions being very insensitive to shock.

It is a further object of this invention to provide improved liquid monopropellant compositions based on difluoroamino-substituted compounds as fuels and dinitrogen tetraoxide as the oxidizer, said compositions being very insensitive to shock.

Another and still further object of the invention is to provide a method of utilizing difluoroamino-substituted compounds as fuels and dinitrogen tetraoxide as an oxidizer in a liquid bipropellant system.

The manner in which these and other objects may be accomplished will become apparent from the detailed 7 description given hereinbelow.

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Many liquid difluoroamino derivatives which are suitable as fuels have been synthesized. All these derivatives thus far testedhave proven to be much less sensitive to shock when in solution with the dinitrogen tetraoxide oxid-izers. Especially suitable liquid difluoroamino derivatives are those prepared by the reaction of an ethylenically unsaturated compound with tetrafluorohydrazine. This reaction results in the addition of a mole of the tetrafiuorohydrazine to the ethylenic linkage, thus producing a compound characterized by the presencec of the group Among the unsaturated compounds which are suitable for the synthesis of difluoroamino-substituted compounds by this addition reaction are ethylene, propylene, 4-methyl-pentene-l, butene-1, butene-2, styrene, allyl alcohol, divinyl ether, methyl a-crylate, vinyl acetate, cyclohexene, and furan.

A preferred class of difluoroamino derivatives suitable as fuels in the propellant compositions of the invention are the tetrafluorohydrazine adducts derived from alkenes of up to 8 carbon atoms. These adducts correspond to the formula wherein R, R, R", and R' are selected from the group consisting of hydrogen and lower alkyl radicals. The lower alkyl radicals can contain up to four carbon atoms and be straight or branched chain. However, because of solubility in the dinitrogen tetraoxide and propellant performance the total number of carbon atoms in the molecule should not exceed eight. Therefore, R, R, R", and R together should not exceed six carbon atoms. Within this preferred class of compounds, the tetrafluorohydrazine adduct of isobutylene is an especially preferred embodiment. In this adduct R and R are methyl radicals while R" and R are hydrogen.

This preferredclass of difluoroamino derivatives appear to be miscible in all proportions with the dinitrogen tetraoxide oxidizer.

-The ratio of dinitrogen tetraoxide to the tetrafluorohydrazine adduct of the alkenes can vary from .60 to 1.2 times the stoichiometric amount. The stoichiometric amount as used hereinafter is that amount of dinitrogen tetraoxide required to insure complete combustion of the particular adduct or adducts, arbitrarily assuming that the only products of combustion are hydrogen fluoride, carbon dioxide, nitrogen, and water. Thus, for example, the stoichiometric molar ratio of dinitrogen tetraoxide to the tetrafiuorohydrazine adduct of isobutylene is 2.5 :l.0. This is shown by the following equation.

a 5N2o 2HzCCH- CH2 8002 7N: sHF 4H2o NFz NF:

In this same manner, the stoichiometric molar ratio of dinitrogen tetraoxide to the adduct of propylene is 1.75:1.0 as shown below.

7M0, 4H3CCHCHg 12002 11N1 16 HF 41120 NF: NFz

It is apparent to one skilled in the art that there are 0 many other possible combustion products such as H furnishes a specific base from which the preferred ratios of dinitrogen tetraoxide to tetrafluorohydrazine adduct may be accurately defined.

The specific ratio of dinitrogen tetraoxide to tetrafiuorohydrazine employed in a particular propellant composition is dependent upon many variables. For example, the particular motor employed may require a composition slightly rich in fuel to give optimum performance in which case an amount of oxidizer somewhat less than the stoichiometric amount would be employed. Again, a composition which has an amount of dinitrogen tetraoxide in excess of the stoichiometric amount may be necessary to achieve optimum performance with a different type motor. The determination of the most desirable ratios of dinitrogen tetraoxide to tetrafiuorohydrazine adduct for any particular rocket motor Within the'bro'ad range set forth above is a matter of routine experimentation easily within the skill of the art. The broad ratio of dinitrogen tetraoxide to adduct of .6 to 1.2 times the stoichiometric amount does provide operable propellant compositions from which optimum ratios can be determined.

The propellant composition found in actual test to yield the highest impulse with l,2-bis(difluoroamino)-2-methyl-propane, the adduct of isobutylene, was a solution of a molar ratio of dinitrogen tetraoxide to adduct of about 15:10. This corresponds to about .6 times the stoichiometric amount. Calculations of shifting specific impulse were made on this solution, giving a specific impulse of 293.56 lb.sec./lb. at 1000 p.s.i. expanded to sea level pressure.

When the dinitrogen tetraoxide and the alkene adducts are to be utilized in a mono-propellant composition, the required amounts of the tetrafiuorohydrazine adduct and the dinitrogen tetraoxide are simply added together in the liquid state to form a solution. As mentioned before the tetrafluorohydrazine adducts of the alkenes appear to be completely miscible with dinitrogen tetraoxide.

If it is desirable to employ the dinitrogen tetraoxide and tetrafluorohydrazine adducts as components in liquid bipropellant systems, the requisite amounts of oxidizer and fuel which produce optimum performance in the particular motor are determined in the conventional manner. The ratio of dinitrogen tetraoxide to tetrafluorohydrazine adduct injected into the combustion zone of the bipropellant system can vary from .6 to 1.2 times the stoichiometric amount, the stoichiometric amount being as previously defined. In preparing the components for use in the bipropellant system, a portion of the predetermined quantity of dinitrogen tetraoxide is added to the tetrafluorohydrazine adduct. Thus, the fuel component of the bipropellant system consists of a mixture of dinitrogen tetraoxide and the adduct. This fuel is brought together in the combustion zone with the additional dinitrogen tetraoxide oxidizer required to furnish the remainder of the predetermined quantity of oxidizer mentioned hereinabove. Since the components are not hypergolic, the combustion chamber must have means for igniting the components. Generally, the molar ratio of dinitrogen tetraoxide to the alkene adduct in the fuel component of the bipropellant system will be in the range of 0.04:1.0 to 05:10. The quantity of dinitrogen tetraoxide in solution with the tetrafiuorohydrazine adduct serves as a desensitizing agent for the adduct before combustion and together with the additional dinitrogen tetraoxide, which is also injected into the combustion zone, functions as an oxidizer.

As in the case of the monopropellant compositions, the preferred components for the bipropellant rocket engines are dinitrogen tetraoxide as the oxidizer and 1,2-bis(difluoroamino)-2-methyl-propane as the fuel. The molar ratio of the dinitrogen tetraoxide to the 1,2-bis(difiuoroamino)-2-methyl-propane in the combustion zone, inclusive of the dinitrogen tetraoxide in admixture with the fuel component is about 15:10. The amount of the oxidizer that is mixed with the fuel component prior to in- 4% jection in the combustion zone can vary from the previously mentioned molar ratio of 0.04: 1.0 to 05:10 while the remaining dinitrogen tetraoxide necessary to raise the ratio of oxidizer to fuel to about 1.5: 1.0 is added in the combustion zone.

The propellant compositions of the invention are suitable for employment in conventional rocket motors now employing other liquid monoor bipropellant compositions. The conventional ignition systems perform satisfactorily in the large motors while in small scale laboratory work, the hot wire type of ignition may be used. One suitable method for igniting the propellants consists of igniting a butane-air mixture which then ignites the propellant mixture. Ignitions sytems for liquid propellants are well known to those skilled in the art and need no detailed description here.

In the preparation and utilization of the liquid propellants of the invention, the low boiling point (213 C.) of dinitrogen tetraoxide should be considered. Obviously working at temperatures below this point will prevent undue loss of the material and facilitate maintaining the desired ratio of components.

The propellant compositions of the invention are not restricted to those containing only dinitrogen tetraoxide and a single difluoroamino derivative but are also intended to include solutions of two or more difiuoroamino derivatives in dinitrogen tetraoxide. For example, the tetrafiuorohydrazine adducts of isobutylene and propylene can be combined and utilized with dinitrogen tetraoxide according to the invention, Furthermore, the compositions may contain small amounts of burning rate modifiers and other conventional additives without departing from the invention.

In addition to being very resistant to shock, the components of the propellant compositions exhibited very good storage characteristics. Various ratios of dinitrogen tetraoxide were subjected to temperatures up to 100 C. At C. to C. there appeared to be an extremely slow reaction with prolonged heating. However, below this point there does not appear to be any reaction or decomposition of the propellant compositions. At 50 C. prolonged standing did not result in nay detectable reaction or decomposition. The excellent compatibility of the fuels and oxidizer of the propellant composition of the invention render these propellants especially suitable for military application where rocket propellants may be subjected to prolonged storage.

Adiabatic compression tests on the Olin-Mathieson Drop-Weight Tester Were conducted to determine the shock sensitivity of the mono-propellant compositions compared to the shock sensitivity of the difiuoroamino derivatives. This is a standard test approved by the Joint Army-Navy-Air Force Panel on Liquid Propellant Test Methods. Essentially, the test comprises subjecting the material to be tested to the shock of a known weight falling a known distance. The size of the weight and the length of the fall is varied until a combination of a certain weight falling at particular distance produces detonation in 50% of the samples tested in a reasonably large number of tests. Arbitrarily, the. weight is measured in kilograms while the distance the weight is dropped is measured in centimeters. The values obtained from such tests, that is, the E values, are expressed in kilogram-centimeters. For example, if a 1.0 kilogram weight dropped 50 centimeters produces 50% detonations in the samples tested, that composition has a test result of 50 kg.-cm. Another sample requiring a 2.0 kilogram weight falling 50 centimeters would have a value of 100 kg.-cm. The higher the kg.-cm. value, the less sensitive to shock is the material being tested.

In testing the pure isobutylene adduct, an E value of 6.9 was found. With a solution of dinitrogen tetraoxide and the adduct of isobutylene, a molar ratio of 0.056: 1.0 gave an E value of 47 kg.-cm. while a molar ratio of 0.56: 1.0 gave an E value of 103 kg.-cm.

As previously mentioned, useful difluoroamino derivatives are those obtained by the addition reaction of tetrafluorohydrazine with an ethylenically unsaturated compound. This reaction has broad applicability, most ethylenically unsaturated compounds entering readily into the reaction. The reaction is generally represented as follows:

The above reaction can be conducted under a wide range of conditions. Since one mole of tetrafluorohydrazine is required for each double bond entering into the reaction, at least one mole of tetrafiuo-rohydrazine should be employed per double bond. Generally, an excess of the tetrafluorohydrazine will increase the yield and/ or shorten the reaction time. A large excess of tetrafiuorohydrazine, for example, moles or more, is not detrimental to the reaction. The reaction temperature. can vary from 40 C. to 150 C. Moreover, the process is operable at pressures ranging from sub-atmospheric to super-atmospheric pressures. Ordinarily the pressure will vary from 100 mm. to 5 atmospheres. The reaction time is obviously dependent on other reaction conditions. Thus, the particular ethylenically unsaturated reactant, the ratio of reactants, the temperature of the reaction, and the pressure will all alfect the time of reaction. Therefore, the reaction time will normally range from two hours to fifteen hours depending on the particular reaction cond-itions.

The examples given hereinbelow are illustrative of the general method of reacting ethylenically unsaturated compounds with tetrafluorohydrazine. The reaction is illustrated with alkenes since the adducts of the alkenes represent the preferred class of difiuoroamino derivatives for the propellant compositions of the invention. However, by substituting for the alkenes other ethylenically unsaturated derivatives such as styrene, methyl acrylate, and divinyl ether, additional difiuoroamino derivatives suitable for use in the liquid monopropellant compositions of the invention can be prepared.

Example I.Preparation of l,2-bis(difluoroa=mino) ethane There was introduced into a 500 cc. evacuated Pyrex bulb equipped with a stopcock and condensing arm 8.7 mole crude tetrafluorohydrazine (75% N F remainder being NF N 0, and NO) and 6.7 10 mole ethylene. The bulb was then attached to a capillary monometer and heated for fifteen hours at a temperature of 110 C. to 120 C. in an oil bath. During this period of time the pressure in the bulb decreased from 655 mm. to a pressure of 435 mm., both pressures being measured at 110 C. The bulb was removed from the oil bath and the condensing arm immersed in a bath at -19'6 C. A pressure of about 3 mm. of noncondensable material was pumped from the bulb. The material remaining in the bulb was warmed to room temperature and transferred into the vacuum system. A small quantity of nonvolatile material remained in the reaction bulb. The material in the vacuum system was fractionated through traps maintained at 78 C., 110 C., 160 C., and -196 C. The fractions at --l60 C. and 196 C. were combined and found to represent 4.8x l0- moles. The percentage composition of this combined fraction was determined by mass spectral analysis. Eight-eight percent of the tetrafluorohydrazine had reacted, the ratio of tetrafluorohydrazine to ethylene consumed in the reaction was 1.16. The NO present as an impurity had reacted while the amounts of NE; and N 0 remained virtually unchanged.

Thereafter, the 78 C. and 110 C. fractions were combined and refractionated for three hours through traps maintained at 46 C., 66 C., 9 6 C., and -196 C. The major fraction was found in the -96 C. fraction as a colorless liquid with a vapor pressure of 32 mm. at 0 C. The same pressure was rapidly obtained by warming the sample from 96 C. or cooling the sample from room temperature, indicating the fraction was homogenous. A gas density molecular weight determination on an aliquot of the fraction gave a value of 130.3. The molecular weight of 1,2-bis(difiuoroamino)-ethane would be 132.1. The yield was 55% based on the tetrafluorohydrazine consumed.

Calculated for C H N F C, 18.19; H, 3.05; N, 21.21; F, 57.55. Found: C, 18.60; H, 3.29; N, 20.55; F, 53.9.

Example II.--Prepa'ration of 1,2-bis(difluoroamino) propane The apparatus was the same as employed in Example I. The reaction bulb was charged with 6.4x 10* mole crude N F and 61x10" mole propylene. The bulb was heated in an oil bath at 100 C. for eight hours. During this interval, the pressure in the bulb decreased from 518 mm. to 316 mm, both pressures being measured at 106 C. The reaction mixture was fractionated essentially as described in Example I. The major product, collected at 96 C., was a colorless liquid with a reproducible vapor pressure of 21mm. at 0 C. A gas density molecular weight determination on an aliquot gave a value of 146.9; 1,2-bis(difluoroamino)-propane requires 146.1. The fraction weighed .45 gram representing an approximate yield of 60% based on the consumption of tetrafluorohydra'zine.

Calculated for C H N F C, 24.66; H, 4.14; N, 19.18; F, 52.02. Found: C, 24.72; H, 4.74; N, 18.05; F, 49.9.

Example 1II.-Preparation of 1,2-bis(difiuoroamino)- propane In a 500 cc. stainless steel bomb, a 1.5 :1 molar ratio of tetrafiuorohydrazine to propylene was heated at 110 C. for two and one-half hours. The pressure decreased during the period of heating from 72 psi. to 21 p.s.i., both pressures being measured at 25 C. The product was distilled through a Holtzman column under an atmosphere of nitrogen yielding about six grams of 1,2-bis (difiuoroamino)-propane which represented a yield of approximately based on the amount of propylene employed in the preparation.

From a comparison of Example II with Example III, it is seen that pressure can greatly facilitate the addition reaction.

Example IV.Preparation of 1,2-bis(difluoroamino)-2- methyl-propane To an evacuated 250 cc. Pyrex bulb was added 1.27 moles of tetrafiuorohydrazine and 1 mole of isobutylene. The bulb was heated in an oil bath for six hours at a temperature of 110 C. The product was purified by bulb to bulb distillation which resulted in the recovery of 3.6 g. of the liquid product. This represented approximately an 86% yield. The micro-HP. for the product was 99.5 C. to C. Various reproducible vapor pressures exhibited at dilferent temperatures by the product are as follows. P01) c =12.1 111111.; P151) mm.; P2730 C =49.3 mm. A quantitative hydrolysis of 61.06 mg. of the product in 35.99 ml. of 0.1111 N N OH-l0 ml. ethanol solution for twenty-one hours gave an equivalent weight of 40.14. The calculated equivalent weight is 40.0.

Calculated for C H N F F, 47.5%. Found: F, 47.2%.

Other alkenes such as butene-l, butene-Z, and 4-methylpentene-l can be substituted for the ethylene, propylene, and isobutylene of Examples I-IV to prepare the corresponding tetrafluorohydrazine adducts.

While the present invention has been illustrated in considerable detail in the foregoing discussion, these details are not intended to be construed as limitations thereof. Many apparent modifications falling within the spirit and 7 scope of the invention will be obvious to those skilled in the art.

I claim: 1. A liquid monopropellant composition comprising a solution of dinitrogen tetraoxide and at least one difluoroamino-substituted compound of the formula wherein R, R, R, and R' are each members selected from the group consisting of hydrogen and lower alkyl groups, said alkyl groups having up to four carbon atoms, the total number of carbon atoms in R, R, R, and R' not to exceed six, the ratio of dinitrogen tetraoxide to said difiuoroamino-substituted compound being from .6 to 1.2 times that of the stoichiometric amount required to furnish sufiicient oxygen to convert the carbon in said difiuoroamino-substituted compound to carbon dioxide.

2. The composition according to claim 1 wherein R is methyl and wherein R, R, and R are hydrogen.

3. The composition according to claim 1 wherein R and R are methyl and wherein R" and R are hydrogen.

4. A liquid monopropellant composition comprising a solution of dinitrogen tetraoxide and 1,2-bis(difiuoroamino)-2-methyl-propane; the molar ratio of dinitrogen tetraoxide to 1,2-bis(difiuoroamino)-2-methyl-propane being approximately 1.5 :1.0.

5. A method of operating a rocket engine which comprises bringing together and igniting in the combustion zone of said engine (1) a mixture of dinitrogen tetraoxide and difiuoroamin-o-substituted compound of the formula 6. The method according to claim 5 wherein R is methyl and R, R, and R are hydrogen.

7. The method according to claim 5 wherein R and R are methyl and R and R are hydrogen.

8. The method of operating a rocket engine which comprises bringing together and igniting in the combustion zone of said engine (1) a mixture of dinitrogen tetraoxide and 1,2-bis(difluoroamino)-2-methyl-propane, the molar ratio of dinitrogen tetraoxide to 1,2-bis(difiuoroamino)-2- methyl-propane being from 0.04210 to 0.5:1.0; and (2) additional dinitrogen tetraoxide such that the total ratio of dinitrogen tetraoxide to l,2-bis(difluoroamino)-2-methylpropane in the combustion chamber inclusive of that in admixture with said 1,2-bis(difluoroamino)-2-methyl-propane is .6 times the stoichiometric amount.

References Cited UNITED STATES PATENTS 3,310,444 3/1967 Gould 14974 3,345,821 10/1967 Magee 60-2l7 XR BENJAMIN R. PADGETT, Primary Examiner.

CARL D. QUARFORTH, Examiner. 

5. A METHOD OF OPERATING A ROCKET ENGINE WHICH COMPRISES BRINGING TOGETHER AND IGNITING IN THE COMBUSTION ZONE OF SAID ENGINE (1) A MIXTURE OF DINITROGEN TETRAOXIDE AND DIFLUOROAMINO-SUBSTITED COMPOUND OF THE FORMULA 